The invention concerns a one-tube or two-tube shock-absorbing leg for a semi-active chassis in a motor vehicle. The leg consists of a cylindrical tube demarcated by a base at the bottom and by a piston-rod aperture at the top. The base of the two-tube embodiment has a valve. A piston is attached to the piston rod. The piston contains valves. The piston divides the cylindrical tube into two force-transmission chambers. The leg also has a spring plate that accommodates a helical spring. The plate is mounted on the cylindrical tube in the one-tube embodiment and on an outer tube that surrounds the cylindrical tube in the two-tube embodiment. A traction limiter above the piston on the piston rod demarcates how far the wheel can emerge. Shock-absorbing legs of this type are employed as linkages to accommodate effective bending moments and to transmit vibrations from the surface of the road to the body of the vehicle by way of the suspension system, especially that of the forward axle. To ensure a comfortable ride and increase safety, shock-absorbing legs with semi-active or active vibration-suppression controls are often employed. For this type of control it is necessary to measure a wide range of motion variables such as the speed of the axis in relation to that of the body or the acceleration of the body and to forward them to controls that emit a parameter for varying the level of vibration suppression. German Patent 3 909 190 describes a relative-speed sensor for a dashpot. The sensor consists of a cylindrical coil accommodated in a jacket attached to the piston rod and of a permanent magnet at the end of the cylinder that the piston rod extends through. The voltage induced in the sensor's coil is employed as an indication of relative speed. Although such a sensor will operate very precisely in a dashpot, it would when employed in a shock-absorbing leg lead to false measurements that would make regulation impossible. This is because the helical spring mounted on the spring plate in a shock-absorbing leg also acts as a coil. The motion of the spring in relation to the sensor coil and the varying width of the spring windings would accordingly generate inductive interference that could not be compensated for by electronic processing. Shock-absorbing legs also take up too much space to allow the use of jackets or protective rubber bellows that would accommodate a state-of-the-art sensor winding.